nir: Add a way to identify per-primitive variables

Per-primitive is similar to per-vertex attributes, but applies to all
fragments of the primitive without any interpolation involved.

Because they are regular input and outputs, keep track in shader_info
of which I/O is per-primitive so we can distinguish them after deref
lowering.  These fields can be used combined with the regular
`inputs_read`, `outputs_written` and `outputs_read`.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10600>

src/compiler/shader_info.h

@@ -154,6 +154,12 @@ typedef struct shader_info {
    /* Which system values are actually read */
    BITSET_DECLARE(system_values_read, SYSTEM_VALUE_MAX);
 
+   /* Which I/O is per-primitive, for read/written information combine with
+    * the fields above.
+    */
+   uint64_t per_primitive_inputs;
+   uint64_t per_primitive_outputs;
+
    /* Which 16-bit inputs and outputs are used corresponding to
     * VARYING_SLOT_VARn_16BIT.
     */